1. Field of the Invention
The present invention relates to an assembling method and an assembling apparatus for a nuclear fuel assembly which is used in nuclear reactors such as a pressurized water reactor.
2. Conventional Art
An existing conventional nuclear fuel assembly is illustrated in FIG. 6.
In FIG. 6, numerals 1 and 2 respectively designate a top nozzle and a bottom nozzle which are positioned apart from each other in a vertical direction and disposed so as to face each other. A plurality of control-rod guide pipes 3 are extended between and securely fixed to the top and bottom nozzles 1, 2. A plurality of supporting grids 4 are secured to intermediate portions of the respective control-rod guide pipes 3 such that they are vertically positioned apart from each other.
As illustrated in FIGS. 7 to 9, the foregoing supporting grid 4 is formed such that a plurality of straps 7 are assembled perpendicularly to each other into a grid form by the mutual fitting of slits 8 which are formed and equally spaced in the longitudinal direction in the strap, and consequently, a plurality of grid cells 5 are formed in each grid. On the walls of the prescribed grid cells 5, there is provided a pair of dimples 9 and springs 10 which are arranged to face each other, for holding a fuel rod 6. The fuel rod 6 inserted in the grid cell 5 is held in the grid 5 by being pressed against the dimples 9 by the springs 10.
According to the conventional method for inserting the fuel rod 6 into the grid cell 5 in the above-mentioned fuel assembly, a plurality of supporting grids 4 (normally, eight supporting grids) of which grid cells 5 are directed in a horizontal direction are disposed into a line in a horizontal direction such that they are disposed apart from each other by the predetermined equal spacing; key members are inserted into the supporting grid 4 so as to deflect the springs 10 away from the dimples 9; and then, a plurality of fuel rods 6 are inserted horizontally into the grid cells 5, thus assembling the fuel assembly.
Meanwhile, the fuel rod generally in use comprises a zircaloy-clad tube having a cylindrical shape of 4 m length or less, and an outer diameter of approximately 10 mm. For this reason, when inserting the fuel rod 6 into the grid cell 5 horizontally in accordance with the foregoing method, the tip of the fuel rod 6 drops down slightly by its own weight such that it may interfere with the supporting grid 4, or that scratches may be formed on the surface of the fuel rod 6 because the lower surface of the fuel rod 6 comes in contact with the supporting grid 4. This is undesirable for producing a fuel assembly of high quality and reliability.